1. Technical Field
The present invention relates to a power conversion device, such as a DC-DC converter, with which it is possible to lengthen the lifespan of a mechanical switch, and reduce loss occurring in a reactor when there is a light load.
2. Background Art
FIG. 7 is a main portion configuration diagram of a heretofore known step-up DC-DC converter. This is shown in, for example, E. K. Sato et al., “Double DC-DC Converter for Uninterruptible Power Supply Applications,” the 2010 International Power Electronics Conference pp. 635-642, 2010.
FIG. 7 is a DC-DC converter that, converting a voltage (=an input voltage Vin) of a direct current power source Vs to a voltage (=an output voltage Vout) higher than the voltage of the Vs by alternately turning on and off a TP and a TN, which are switching elements (in the drawing, insulated gate bipolar transistors: IGBTs), at an appropriate time ratio, supplies power to a load.
Also, in FIG. 7, the load is a current source Load assumed to be an automobile motor, or the like, but even when the direction of a current ILoad of the current source Load is a direction opposite to that shown in the drawing, the TP and TN are turned on and off at an appropriate time ratio so that the voltage (=the output voltage Vout) applied to the load (=the current source Load) is a desired voltage. In this case, a current IL of a reactor L flows in a direction opposite to the direction shown in the drawing, and power is regenerated from the load (=the current source Load: a motor or the like) to the direct current power source Vs. In this way, in the case of the circuit configuration of FIG. 7, a bidirectional power flow (bidirectional flow), from the direct current power source Vs to the current source Load or from the current source Load to the direct current power source Vs, is possible. SW in the drawing, being a mechanical switch, breaks (turns off) the circuit when there is a problem with the DC-DC converter. The mechanical switch is widely used, as it is resistant to noise and is low cost. Also, the mechanical switch being a contact switch, it may be a knife switch, a breaker, a relay switch with contact, or the like, with the insertion into and removal from a socket of a plug also included. Although the mechanical switch is not directly shown in the E. K. Sato et al. article, it is indispensible for an actual device.
FIG. 8 is a configuration diagram of another heretofore known step-up DC-DC converter. While a bidirectional power flow is possible in FIG. 7, there are cases in which, depending on the load, it is sufficient that the power flow is in one direction, that is, it is sufficient to supply power to the load (a unidirectional flow). In this case, it is possible to omit the TP of FIG. 7. Also, with the current IL flowing through the reactor L flowing in the direction of the arrow in FIG. 8, the power flow and current directions are unidirectional.
FIG. 9 is a configuration diagram of a heretofore known step-down DC-DC converter. While FIG. 8 is a step-up DC-DC converter that supplies power to the load by boosting the voltage of the direct current power source Vs, the circuit configuration of FIG. 9 is a step-down DC-DC converter that supplies power to the load by reducing the voltage of the direct current power source Vs. In this case, it is possible to omit the TN of FIG. 7. Also, with the current IL flowing through the reactor L flowing in the direction of the arrow in the drawing, the power flow and current directions are unidirectional (a unidirectional flow).
Also, in JP-A-2007-213842, it is described how an arc is prevented by providing an electronic switch in series with a mechanical switch, and arranging that no high voltage is applied to the mechanical switch by first turning off the electronic switch, then turning off the mechanical switch.
Also, in JP-A-2007-252164, it is described how a resonance circuit is connected in parallel with a mechanical switch, the sum of the normal current of a direct current power source system and a resonance current is caused to flow through the mechanical switch, and the mechanical switch is turned off when the sum is zero.
In the circuits shown in FIG. 7, FIG. 8, and FIG. 9, the mechanical switch SW is for electrically disconnecting the direct current power source Vs or the load (the current source Load) and the power conversion device, and acts (goes off) when a problem occurs with the power conversion device, or when a problem occurs with the load.
Herein, as it is necessary for the mechanical switch SW to consume induction energy accumulated in the reactor L, the allowed value of inductance connected to the mechanical switch SW is, in general, prescribed. Normally, however, even when a reactor L with prescribed inductance is connected, an arc is generated when the mechanical switch SW is turned off (opened), abrading the mechanical switch SW, and shortening the lifespan thereof. Also, when an inductance higher than that prescribed is connected, a problem occurs in that the lifespan of the mechanical switch SW becomes markedly shorter, and the like.
In particular, in FIG. 7 and FIG. 8, all the energy accumulated in the reactor L is consumed by an arc generated when the mechanical switch is turned off. When an arc is generated when the mechanical switch SW is turned off in this way, the contact of the mechanical switch is damaged, and the lifespan thereof is shortened. Also, in FIG. 9, as the energy accumulated in the reactor L is absorbed by the capacitor C when the mechanical switch SW is turned off, it is possible to suppress the shortening of the lifespan of the mechanical switch SW. However, when the capacitance of the capacitor C is small, the voltage of the capacitor C rises markedly due to absorbing the energy accumulated in the reactor L, meaning that it is necessary to arrange that the voltage of the capacitor is equal to or lower than the withstand voltage of the capacitor by using a capacitor with a large withstand voltage, or by using a capacitor with a large capacitance, meaning that the capacitor leads to an increase in size of the device.
Also, although it is assumed that various kinds of load are connected as the load of the power conversion device, in general, the higher the output voltage Vout, the higher the output power of the power conversion device, and conversely, the lower the output voltage Vout, the lower the output power as a power conversion device.
Herein, with a step-up DC-DC converter, it is assumed that driving will be carried out in a condition wherein the output voltage Vout is equal to the input voltage Vin when the load is light (when the device output power is low). Strictly speaking, however, owing to the effect of a semiconductor element (for example, the diode DP) voltage drop, or the like, the output voltage Vout is not necessarily equal to the input voltage Vin.
In this case, in principle, the reactor L is unnecessary. However, there is a problem in that, the reactor L being installed in the circuit, loss occurs due to current flowing through the reactor L.
Also, in JP-A-2007-213842 and JP-A-2007-252164, the circuit added to the mechanical switch SW, and the circuit controlling that circuit, are large-scale, causing a rise in cost. Also, when the output voltage and input voltage are equal when there is a light load, it is not possible to combat a problem of loss of the current IL flowing through the reactor L.